A Rician fading environment is a state in which a wireless communication terminal device that moves at a high speed communicates with a base station apparatus on the line of sight without any obstacles therebetween. In such a Rician fading environment, the Doppler effect on the signals received at each device and apparatus affects the communication quality significantly as a frequency offset.
In particular, a wireless communication terminal device determines the carrier frequency of the uplink signals based on the carrier frequency of the downlink signals received from the base station apparatus. When signals are transmitted from the wireless communication terminal device, the signals include the Doppler frequency that has been generated in the downlink and added as a frequency offset to the signals. Furthermore, when the signals transmitted from the wireless communication terminal device are received at the base station apparatus, the signals include the Doppler frequency that has been generated in the uplink and added to the signals. In light of the foregoing, the base station apparatus preferably have an ability to remove a frequency offset of which effect is no less than twice as large as the effect of the Doppler frequency.
Mobile communications have widely been spread in these years. This requires wireless communication systems to maintain the communications without any interruption if the user moves at a speed of 300 km/h or higher, for example, on the Shinkansen bullet train. Additionally, High frequencies equal to or higher than 2 GHz are increasingly used as the frequency bands for the mobile communications because many frequency bands have been already used. This generates a very high Doppler frequency. Consequently, this causes the base station apparatus to remove a frequency offset of which effect is no less than twice as large as the effect of the very high Doppler frequency. This makes it difficult for the base station apparatuses to receive signals.
A method in which the correlation between reference signals received at different times is calculated and then the amount of phase rotation between the received signals is known as a conventional common method for estimating a frequency offset.
For example, a received signal rk is obtained from the following expression (1) when the signal transmitted at a time k is sk, the distortion caused by a propagation channel is hk, the frequency deviation is Δf, and the white Gaussian noise is nk.rkej2πΔfkhksk+nk phase rotation eτ2πΔfk  (1)
The frequency deviation remains after removal of the carrier wave and thus the phase rotation eτ2πΔfk appears in the expression. In that case, the correlation z(k, τ) between the signal rk received at the time k and the signal rk+τ received at a time k+τ is expressed as the following expression (2).
                                                                        z                ⁡                                  (                                      k                    ,                    τ                                    )                                            =                            ⁢                                                r                                      k                    +                    τ                                                  ⁢                                  r                  k                  *                                                                                                        =                            ⁢                                                                    ⅇ                                          j                      ⁢                                                                                          ⁢                      2                      ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                      Δ                      ⁢                                                                                          ⁢                      f                      ⁢                                                                                          ⁢                      τ                                                        ⁢                                      h                                          k                      +                      τ                                                        ⁢                                      h                    k                    *                                    ⁢                                      s                                          k                      +                      τ                                                        ⁢                                      s                    k                    *                                                  +                                                      ⅇ                                          j                      ⁢                                                                                          ⁢                      2                      ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                      Δ                      ⁢                                                                                          ⁢                                              f                        ⁡                                                  (                                                      k                            +                            τ                                                    )                                                                                                      ⁢                                      h                                          k                      +                      τ                                                        ⁢                                      s                                          k                      +                      τ                                                        ⁢                                      n                    k                    *                                                  +                                                                                                      ⁢                                                                                          n                                              k                        +                        τ                                                              ⁡                                          (                                                                        ⅇ                                                      j                            ⁢                                                                                                                  ⁢                            2                            ⁢                                                                                                                  ⁢                            π                            ⁢                                                                                                                  ⁢                            Δ                            ⁢                                                                                                                  ⁢                            fk                                                                          ⁢                                                  h                          k                                                ⁢                                                  s                          k                                                                    )                                                        *                                +                                                      n                                          k                      +                      τ                                                        ⁢                                      n                    k                    *                                                                                                          (        2        )            
On the assumption that the propagation channel does not vary during a signal interval τ, and the transmitted signals sk and sk+τ are the same, the average of the correlations z(k, τ) is zero in and after the second term due to the characteristic of the white Gaussian noise. The following expression (3) can be obtained from the above.E[z(k,τ)]=ej2πΔfr  (3)
The frequency deviation Δf can be estimated as the following expression (4) according to the results described above. Note that, if the transmitted signals sk and sk+τ are predetermined, the frequency deviation Δf can similarly be estimated after a simple deformation of the expression.
                              Δ          ⁢                                          ⁢          f                =                                            arg              ⁡                              (                                  E                  ⁡                                      [                                          z                      ⁡                                              (                                                  k                          ,                          τ                                                )                                                              ]                                                  )                                                    2              ⁢                                                          ⁢              π              ⁢                                                          ⁢              r                                =                                    1                              2                ⁢                                                                  ⁢                π                ⁢                                                                  ⁢                r                                      ⁢                                          tan                                  -                  1                                            ⁡                              [                                                      Im                    ⁡                                          (                                              E                        ⁡                                                  [                                                      z                            ⁡                                                          (                                                              k                                ,                                τ                                                            )                                                                                ]                                                                    )                                                                            Re                    ⁡                                          (                                              E                        ⁡                                                  [                                                      z                            ⁡                                                          (                                                              k                                ,                                τ                                                            )                                                                                ]                                                                    )                                                                      ]                                                                        (        4        )            
To prevent any high Doppler frequency from deteriorating the estimation accuracy, a conventional technique that increases the number of reference signals to be added to a Physical Uplink Shared Channel (PUSCH) that is uplink data signals is used when the improvement effect of the increase on the deterioration is confirmed. There is also a conventional technique that creates a reference signal transmission schedule suitable for estimating the highest Doppler frequency.    Patent Document 1: Japanese Laid-open Patent Publication No. 2011-77647    Patent Document 2: Japanese Laid-open Patent Publication No. 2011-193124
However, the range of the inverse tangent function is −π<tan−1 x<π in the conventional estimating methods, and thus the range in which the frequency deviation Δf can be estimated is limited to −½τ<Δf<½τ in the conventional estimating methods. The range in which the frequency deviation can be estimated equates with the range in which the frequency offset can be estimated. This means that the range in which the frequency offset can be estimated is determined depending on the signal interval τ. For example, when the signal interval τ is 1 ms, the frequency offset can be estimated in the range in of ±500 Hz.
As indicated in the expression (4), the angle fails to be estimated as a unique angle when the signal rotates one or more revolutions along the circumference. In other words, when the amount of phase rotation between the reference signals received at the signal interval τ is estimated as a value θ, it fails to be determined whether the reference signal has rotated by θ at a low speed during the signal interval τ, or the reference signal has rotated by θ+2π at a higher speed.
For example, a reference signal at intervals of 500 μs is transmitted in the PUSCH in the Long Term Evolution (LTE), and thus the frequency offset can be estimated in the range of ±1000 Hz. The interval between the reference signals is 285.417 μs in the Physical Uplink Control Channel (PUCCH) that is the uplink control signals in the LTE, and thus the frequency offset can be estimated in the range of ±1751 Hz. Consequently, it is difficult to estimate the frequency offset in the range of 2000 Hz and higher in the LTE with PUSCH or PUCCH.
If a frequency offset is estimated in a wide range by combining PUSCH and PUCCH, the combination fails to be used to estimate the frequency offset in a cell to which the PUCCH is not transmitted, for example, in the Secondary Cell (SCell) in the Carrier Aggregation (CA).